1. Field of the Invention
The present invention relates to a black ink composition for ink jet recording which can realize, even on plain papers, printed images having high quality and high color density and, at the same time, having excellent lightfastness and waterfastness.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink composition are ejected and deposited onto recording media, such as paper, to conduct printing. This method has a feature that images having high resolution and high quality can be printed at a high speed by means of relatively inexpensive apparatuses. In general, the ink composition used in the ink jet recording comprises water as a main component and, incorporated therein, a dye or a pigment as a colorant. and a wetting agent, such as glycerin, for preventing clogging and other purposes.
Further, in ink jet printers, good waterfastness and lightfastness are required of printed images from the viewpoint of enhancing the general-purpose properties of the ink jet printers. In order to improve the waterfastness of the printed images, Japanese Patent Laid-Open No.140495/1993 proposes an ink composition containing pyrrolidone which is 2-pyrrolidone, N-(2-hydroxyethyl)-2-pyrrolidone, or a mixture of 2-pyrrolidone with N-(2-hydroxyethyl)-2-pyrrolidone. Further, U.S. Pat. Nos. 4,963,189 and 5,203,912 disclose dyes which can be easily dissolved in bases, but are insoluble in water. These dyes have waterfastness which has been improved to some extent. However, there is still room for improvement in waterfastness. This is true of lightfastness.
Use of pigments as the colorant for satisfying both waterfastness and lightfastness requirements is well known in the art. In general, however, as compared with dyes, the pigments are likely to have inferior color development and to provide lower optical density.
In recent years, there is an increasing demand for high-speed printing. In order to realize high-speed printing, rapid fixation of ink onto recording media is necessary. This has lead to an attempt to improve the penetration of the ink into recording media. For example, Japanese Patent Laid-Open No. 183761/1992 and U.S. Pat. Nos. 5,156,675 and 5,183,502 disclose, for example, the addition of diethylene glycol monobutyl ether and the addition of a glycol ether in combination with a nonionic acetylene glycol surfactant.
In the highly penetrable ink using the pigment, however, the pigment to be stayed on the surface of the recording medium is more likely to be penetrated into the recording medium, due to the improved penetration, and to have poor color development. In particular, for plain papers not subjected to any special surface treatment (for example, copying papers, reporting papers, bond papers, letter papers, postcards, and slip sheets, used in offices, schools, households and the like), printed images were observed to have remarkably lowered optical density.